They were looking for gold. Prospectors were all over Montana in the mid-19th century, finding both minerals and gemstones, but since they were solely focused on gold, they overlooked the best sapphires in the U.S. They threw away the blue stones that showed up in their pans at Yogo Gulch, not realizing that they were worth more than the gold they were searching for. That changed in 1895, when a prospector sent a box of blue stones to Dr. George F. Kunz, a gemologist at Tiffany's. Sapphires of various colors from Montana were common, but those from the Yogo Gulch were special.
In 1897, Kunz wrote for the American Journal of Science, and detailed the specific and ultimate coloration of sapphires from the Yogo Gulch region. He wrote that the deviation in color of the stones were “varying from light blue to quite dark blue, including some of the true ‘cornflower’ blue tint so much prized in the sapphires of the Ceylon… Some of them are ‘peacock blue’ and some dichroic, showing a deeper tint in one direction than in another; and some of the ‘cornflower’ gems are equal to any of the Ceylonese, which they strongly resemble,—more than they do those of the Cashmere.”